Conventional oscillators and voltage controlled oscillators (VCOs) used in integrated circuit (IC) designs are susceptible to phase or frequency error and are limited in frequency range due to instability across a wide bandwidth. Commonly used ring oscillators are notoriously susceptible to phase or frequency error, altering the resonant frequency proportional to any phase error induced in the loop. Phase error can come from a number of sources, but a dominant source is noise on voltage supplies causing jitter and hence phase error on circuit elements in the loop.
Conventional approaches to deal with phase noise have been orientated toward controlling noise sources rather than designing oscillators to be more immune to such noise. One exception are oscillators that use higher-order filters to lock in frequency independent of phase error. But such conventional oscillators are expensive to implement, requiring more accurate discrete components, which is not amenable to IC design. Also, tuning to a tight band may not be useful in applications requiring wide frequency ranges, such as VCOs or oscillators requiring tuning across frequency.